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The country is in the grip of three emerging flu or flulike epidemics: an early start to the annual flu season with an unusually aggressive virus, a surge in a new type of norovirus, and the worst whooping cough outbreak in 60 years. And these are all developing amid the normal winter highs for the many viruses that cause symptoms on the "colds and flu" spectrum.

Influenza is widespread, and causing local crises. On Wednesday, Boston's mayor declared a public health emergency as cases flooded hospital emergency rooms.

Google's national flu trend maps, which track flu-related searches, are almost solid red (for "intense activity") and the Centers for Disease Control and Prevention's weekly FluView maps, which track confirmed cases, are nearly solid brown (for "widespread activity").

"Yesterday, I saw a construction worker, a big strong guy in his Carhartts who looked like he could fall off a roof without noticing it," said Dr. Beth Zeeman, an emergency room doctor for MetroWest Medical Center in Framingham, Mass., just outside Boston. "He was in a fetal position with fever and chills, like a wet rag. When I see one of those cases, I just tighten up my mask a little."

Massachusetts General Hospital in Boston started asking visitors with even mild cold symptoms to wear masks and to avoid maternity wards. The hospital has treated 532 confirmed influenza patients this season and admitted 167, even more than it did by this date during the 2009-10 swine flu pandemic.

At Brigham and Women's Hospital, 100 patients were crowded into spaces licensed for 53. Beds lined halls and pressed against vending machines. Overflow patients sat on benches in the lobby wearing surgical masks.

"Today was the first time I think I was experiencing my first pandemic," said Heidi Crim, the nursing director, who saw both the swine flu and SARS outbreaks here. Adding to the problem, she said, many staff members were at home sick and supplies like flu test swabs were running out.

Nationally, deaths and hospitalizations are still below epidemic thresholds. But experts do not expect that to remain true. Pneumonia usually shows up in national statistics only a week or two after emergency rooms report surges in cases, and deaths start rising a week or two after that, said Dr. Gregory A. Poland, a vaccine specialist at the Mayo Clinic in Minnesota. The predominant flu strain circulating is an H3N2, which typically kills more people than the H1N1 strains that usually predominate; the relatively lethal 2003-4 "Fujian flu" season was overwhelmingly H3N2.

No cases have been resistant to Tamiflu, which can ease symptoms if taken within 48 hours, and this year's flu shot is well-matched to the H3N2 strain, the C.D.C. said. Flu shots are imperfect, especially in the elderly, whose immune systems may not be strong enough to produce enough antibodies.

Simultaneously, the country is seeing a large and early outbreak of norovirus, the "cruise ship flu" or "stomach flu," said Dr. Aron J. Hall of the C.D.C.'s viral gastroenterology branch. It includes a new strain, which first appeared in Australia and is known as the Sydney 2012 variant.

This week, Maine's health department said that state was seeing a large spike in cases. Cities across Canada reported norovirus outbreaks so serious that hospitals were shutting down whole wards for disinfection because patients were getting infected after moving into the rooms of those who had just recovered. The classic symptoms of norovirus are "explosive" diarrhea and "projectile" vomiting, which can send infectious particles flying yards away.

"I also saw a woman I'm sure had norovirus," Dr. Zeeman said. "She said she'd gone to the bathroom 14 times at home and 4 times since she came into the E.R. You can get dehydrated really quickly that way."

This month, the C.D.C. said the United States was having its biggest outbreak of pertussis in 60 years; there were about 42,000 confirmed cases, the highest total since 1955. The disease is unrelated to flu but causes a hacking, constant cough and breathlessness. While it is unpleasant, adults almost always survive; the greatest danger is to infants, especially premature ones with undeveloped lungs. Of the 18 recorded deaths in 2012, all but three were of infants under age 1.

That outbreak is worst in cold-weather states, including Colorado, Washington, Wisconsin, Minnesota and Vermont.

Although most children are vaccinated several times against pertussis, those shots wear off with age. It is possible, the authorities said, that a new, safer vaccine introduced in the 1990s gives protection that does not last as long, so more teenagers and adults are vulnerable.

And, Dr. Poland said, if many New Yorkers are catching laryngitis, as has been reported, it is probably a rhinovirus. "It's typically a sore, really scratchy throat, and you sometimes lose your voice," he said.

Though flu cases in New York City are rising rapidly, the city health department has no plans to declare an emergency, largely because of concern that doing so would drive mildly sick people to emergency rooms, said Dr. Jay K. Varma, deputy director for disease control. The city would prefer people went to private doctors or, if still healthy, to pharmacies for flu shots. Nursing homes have had worrisome outbreaks, he said, and nine elderly patients have died. Homes need to be more alert, vaccinate patients, separate those who fall ill and treat them faster with antivirals, he said.

Dr. Susan I. Gerber of the C.D.C.'s respiratory diseases branch, said her agency has not seen any unusual spike of rhinovirus, parainfluenza, adenovirus, coronavirus or the dozens of other causes of the "common cold," but the country is having its typical winter surge of some, like respiratory syncytial virus "that can mimic flulike symptoms, especially in young children."

The C.D.C. and the local health authorities continue to advocate getting flu shots. Although it takes up to two weeks to build immunity, "we don't know if the season has peaked yet," said Dr. Joseph Bresee, chief of prevention in the agency's flu division.

Flu shots and nasal mists contain vaccines against three strains, the H3N2, the H1N1 and a B. Thus far this season, Dr. Bresee said, H1N1 cases have been rare, and the H3N2 component has been a good match against almost all the confirmed H3N2 samples the agency has tested.

About a fifth of all flus this year thus far are from B strains. That part of the vaccine is a good match only 70 percent of the time, because two B's are circulating.

For that reason, he said, flu shots are being reformulated. Within two years, they said, most will contain vaccines against both B strains.

Joanna Constantine, 28, a stylist at the Guy Thomas Hair Salon on West 56th Street in Manhattan, said she recently was so sick that she was off work and in bed for five days — and silenced by laryngitis for four of them.

She did not have the classic flu symptoms — a high fever, aches and chills — so she knew it was probably something else.

Still, she said, it scared her enough that she will get a flu shot next year. She had not bothered to get one since her last pregnancy, she said. But she has a 7-year-old son and a 5-year-old daughter, "and my little guys get theirs every year."